1. Field of the Invention
The present invention relates to an organic device or an element using an organic charge-transporting compound including, for example, an organic electroluminescent device (hereinafter, abbreviated as an “organic EL device”), an organic solar cell, an organic semiconductor device such as organic FET device and others, and a process for the production of such an organic device.
2. Description of the Related Art
Recently, organic semiconductors and organic conductive materials have been actively studied, and in particular, remarkable advances have been achieved in light-emitting organic EL devices which use an organic semiconductor.
Tang et al. have been successful in achieving organic EL devices which have high luminance and high efficiency sufficient for practical application such as a luminance of 1,000 cd/m2 and an external quantum efficiency of 1% at an applied voltage of not more than 10V, if a laminate structure of organic compounds having different carrier transporting properties (organic hole-transporting compounds and organic electron-transporting compounds) are applied to the devices, along with a balanced injection of holes and electrons from an anode and cathode, respectively, and if a thickness of the organic layer sandwiched between the cathode and the anode is controlled to not more than 2,000 Å (cf. Tang et al., Appl. Phys. Lett., 51, 913 (1987); Japanese Patent Application Laid-open Nos. 59-194393, 63-264692 and 2-15595; and U.S. Pat. Nos. 4,539,507, 4,769,292 and 4,885,211).
Moreover, Tang et al. were able to achieve a power conversion efficiency of approximately 1% by laminating organic compounds having different carrier transporting properties (CuPc as an organic hole-transporting compound and PV as an organic electron-transporting compound) in organic solar cells (cf. Tang et al., Appl. Phys. Lett., 48, 183(1986)).
More recently, for the purpose of obtaining a high efficiency in organic devices, various ideas based on the structure such that a plurality of above-mentioned laminated portion (that used to be sandwiched by the electrodes in prior arts) is stacked and connected in series in terms of circuit have been suggested in technical articles and patent literatures (cf. Appl. Phys. Lett., 80, 1667 (2002); Chemistry Letters, pp. 327-330 (1990); Japanese Patent Application Laid-open No. 11-329748; U.S. Pat. No. 6,337,492; and Japanese Patent Application Laid-open Nos. 2003-45676 and 2003-264085).
In particular, the inventor of the present invention has disclosed an serial connection of two or more organic light-emitting EL units in terms of circuit by using an electrically insulating charge generation layer having a resistivity (specific resistance) of not less than 102 Ωcm in Japanese Patent Application Laid-open No. 2003-272860. The inventor of the present invention has named the resulting device an organic MPE (Multi-Photon Emission) EL device, and has disclosed and exhibited, received high evaluation, in many conferences, exhibitions, etc., (cf. 49th lecture meeting, Associate of Society of Applied Physics and others, Preprint 27p-YL-3, p. 1308; 63rd lecture meeting, Society of Applied Physics, preprint 27a-ZL-12, p. 1165; Proceedings of EL2002 (International Conference on the Science and Technology of Emissive Device and Lighting), p. 539; Proceedings of IDMC'03 (International Display Manufacturing Conference), Fr-21-01, p. 413; SID03 DIGEST, Vol. XXXIV, BOOKII, p. 979; 13th lecture meeting, Production Technology Exhibition of Flat Panel Display, D-4(2003); and exhibition and distribution materials, EExpress (Nov. 15, 2002), of white light emitter by IMES Co., Ltd. at LCD/PDP International 2002). Hereinafter, the organic MPE (multi-photon emission) EL device is referred to as a “MPE” device.
The charge generation layer has a structure which is similar to those obtained by laminating, in sequence, the different sorts of charge injection layers (for electron injection and hole injections), which used to be disposed adjacent to the electrodes (cathode and anode) and were invented and have been improved by the inventor of this application. Namely, the charge generation layer is produced by laminating, in sequence, a layer containing radical anion molecules of an organic electron-accepting (electron-transporting) compound produced upon reduction of the electron-accepting compound with a reducing (electron-donating) agent such as alkaline metal, for example, those disclosed in Japanese Patent Application Laid-open Nos. 10-270171 (U.S. Pat. No. 6,013,384) and 2001-102175 (U.S. Pat. No. 6,589,673), and a layer containing radical cation molecules of an organic electron-donating (hole-transporting) compound produced upon oxidation of the electron-donating compound with an oxidizing (electron-accepting) agent such as V2O5, MoO3, and WO3, F4-TCNQ represented by the following formula:
or PNB represented by the following formula:
for example, those disclosed in Japanese Patent Application Laid-open Nos. 11-251067 (U.S. Pat. No. 6,423,429), 2001-244079 (U.S. Pat. No. 6,589,673), 2003-272860 and 2003-358402, and the specification of Patent Application No. 2004-202266. Reference literature: K. L. T. Dao and J. Kido, J. Photopolym. sci. Technol., 15, 261 (2002) (Reference literature: IDW '04 Advance Program, p. 60, OLED2-4, Novel Mg:Alq3/WO3 Connecting Layer for Tandem White Organic Light Emitting Diodes (WOLEDs), C.-C. Chang, S.-W. Hwang, H.-H. Chen, C. H. Chen, J.-F. Chen (2004)).
It has been proved that if the portion, i.e., light-emissive units, which used to be sandwiched by the cathode and anode in the conventional organic EL devices, are stacked via the above-described charge generation layer, light emission intensity per current density (i.e., quantum efficiency or current efficiency, denoted as (cd/A)) are multiplied by approximately (n+1) times (n: the number of charge generation layers), because electrons and holes can move from the charge generation layer to the direction of anode and cathode respectively upon voltage application and recombine with each other in the multiple emissive units, and thus photons can be generated in the multiple light-emissive units.
In this case, since the driving voltage is also increased to approximately (n+1) times, it was expected in theory that the power conversion efficiency cannot be substantially improved or changed.
However, it is appreciated upon more precise and detailed study that when organic MPE EL devices are produced under optimized conditions, it becomes possible to achieve corresponding improvement effects even in the power conversion efficiency. Here, it is well-known that the mobility (denoted as (cm2·V−1·S−1)) of the organic semiconductor is much lower than that of the inorganic semiconductor in a different order, and thus it is necessary to apply an “additional voltage” to the EL devices to obtain the desired current level. The term “additional voltage” used herein means an additionally required voltage to obtain a current density necessary to discharge the desired and much amounts of photons per the unit time, in addition to the device voltage of 2V, for example, which is at least necessary to discharge photons of 2 eV, and in this case, it is represented by the formula: (additional voltage)=(driving voltage−2V). Accordingly, as for an organic EL device in which the luminance is proportional only to the current density, the power conversion efficiency in the high luminance region is made lower than that of the low luminance region, as is appreciated from the graph of FIG. 24.
However, comparing MPE EL devices with conventional EL devices under the same luminance, the MPE devices show a required current density of approximately 1/(n+1), wherein n is a number of the charge generation layers, and thus the potential (voltage) consumed per each light emissive unit is lower than the potential (voltage) consumed in conventional EL devices. Accordingly, in practice a total amount of the electrical power consumption is lowered, i.e., the power conversion efficiency is improved, in MPE devices. FIG. 24 shows the results of the simulation tests in which the light emissive units of the conventional organic EL device were stacked through different numbers of the charge generation layers to make the EL devices having 2 units (n=1), 5 units (n=4) or 10 units (n=9), and the power conversion efficiency (Im/W) of each device was determined as a function of the luminance to ascertain how the power conversion efficiency is varied with the change of luminance. Note in the graph of FIG. 24 that it is the simulated results on the assumption that the current efficiency (cd/A) is exactly increased to (n+1) times wherein n is a number of the charge generation layers, and also the driving voltage is exactly increased to (n+1) times under the application of the same current density (not the same luminance). The plotted line for 1 unit (n=0) in FIG. 24 represents conventional organic EL devices and the data thereof were acquired from the EL device which was actually fabricated.
The MPE devices showing good performances like ones plotted in FIG. 24 cannot be realized with ease. For example, when the voltage required for ‘(n+1) stacked MPE device’ to achieve a current density is larger than (n+1) times the voltage required for the corresponding conventional device (n=0), i.e. in the case that undesirable, “excess voltage (ΔV)” is required every time when the charge generation layer is inserted between the light emissive units, it is appreciated that the power conversion efficiency of the MPE devices is lowered with increase in the stacking number of the light emissive units
FIGS. 25 and 26 each shows a band diagram of the organic MPE EL device having two light emissive units. Undesirable “excess voltage (ΔV)” is generated within the charge generation layer section. Needless to say, but if precisely described, the generation (injection) of holes in the organic EL devices means the withdrawal of electrons from the HOMO (Highest Occupied Molecular Orbital) of the electron-donating molecules (hole-transporting molecules), i.e., formation of radical cation state of the hole-transporting molecules, whereas the generation (injection) of electrons in the organic EL devices means the injection of electrons into the LUMO (Lowest Unoccupied Molecular Orbital) of the electron-accepting molecules (electron-transporting molecules), i.e., formation of radical anion state of the electron-transporting molecules, upon application of voltage.
Accordingly, the role of the charge generation layer in the MPE devices resides in withdrawal of the electrons from HOMO of the hole-transporting molecule in one light-emissive unit appearing on a cathodic side of the charge generation layer and in injection of the electrons (withdrawn from HOMO) into the LUMO of the electron-transporting molecule in another light-emissive unit appearing on an anodic side of the charge generation layer, upon voltage application.
Namely, the “excess voltage (ΔV)” can be ascribed to an “energy barrier” during the electron transfer from the HOMO to the LUMO in the charge generation layer (where hole current is converted to electron current). In this respect, the transfer of the electrons from the HOMO to the LUMO refers to the conversion of the hole current to the electron current.
Accordingly, the above-described MPE devices showing good performance can be achieved by enabling the “excess voltage (ΔV)” to be approached to substantially zero as a result of diminishing the energy barrier for electron transfer within the hole-electron conversion layer to substantially zero, as shown in FIG. 26.
As a result of research and development, the inventor of the present invention has discovered that the ideal “hole current-electron current conversion” could be realized by diminishing the energy barrier to substantially zero. Specifically, it has been discovered that the hole current in the HOMO level can be converted to the electron current in the LUMO level without any energy barrier, if a thermal reduction reaction-generated layer containing radical anion molecules of the electron-accepting organic compound produced upon the thermal reduction reaction according to the methods described in detail in Japanese Patent Application Laid-open No. 11-233262 and Japanese Patent Application Laid-open No. 2000-182774 (U.S. Pat. No. 6,396,209, US Patent Application Publication 20030072967, EP0936844B1, EP1011155B1 as well as J. Endo, T. Matsumoto and J. Kido, Jpn. J. Appl. Phys., Vol. 41(2002) pp. L800-L803), and a layer containing radical cation molecules of the electron-donating organic compound produced through the oxidizing molecules, which is formed according to the methods described in detail in Japanese Patent Application Laid-open Nos. 11-251067 (U.S. Pat. No. 6,423,429), 2001-244079 and 2003-272860 (cf. Endo et al., Jpn. J. Appln. Phys., Vol. 41 (2002) L358, 47th Meeting of Japanese Society of Polymer, Preprint, Vol. 47, No. 9, p. 1940 (1998) and Leo et al., Appl. Phys. Lett., Vol. 78, No. 4, 2001), are stacked in that order.
However, in the case that the radical anion molecules of the electron-accepting organic compound are produced by using the other method also disclosed by the inventors of the present invention, which is described in detail in Japanese Patent Application Laid-open Nos. 10-270171 (U.S. Pat. No. 6,013,384) and 2001-102175 (U.S. Pat. No. 6,589,673) as well as J. Kido and T. Matsumoto, Appl. Phys. Lett., 73, p. 2866 (1998), i.e., the radical anion molecules are produced through direct doping of electron donors (reducing dopants) such as alkaline metals, problems arise such as undesirable reactions occurring unintentionally between the electron donors (reducing agents) and the electron acceptors (oxidizing agents), between the electron-accepting organic compound and the oxidizing agents, and also between the electron-donating molecules (hole-transporting molecules) and the electron donors (reducing agents), to thereby make difficult the transfer of the electrons.
In fact, the inventor has discovered through study and examination of MPE devices that if the above-mentioned technique (i.e., direct doping) is employed for the generation of radical anion state, the “excess voltage (ΔV)” cannot approach to zero. (i.e., the driving voltage exceeds (n+1) times the voltage of the corresponding conventional device (i.e., n=0) at a current density, wherein (n+1) is the number of the light emissive units contained in the MPE device.)
This increasing tendency of the driving voltage is particularly noticeable in a region of the high current density or high luminance, i.e., the excess voltage (ΔV) has a current density dependency. Examples of MPE devices having such undesirable states are disclosed in Japanese Patent Application Laid-open Nos. 2003-45676 and 2003-272860, proposed by the inventors of this application.